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Biological Overview of Chimpanzees

Alicia S. Ivory


Animal Legal and Historical Center
Publish Date:
2007
Place of Publication: Michigan State University College of Law
Printable Version

Biological Overview of Chimpanzees

Biological Account[1]

 

Common Chimpanzee:  Pan troglodytes

 

Physiology

            Chimpanzees stand erect at 1-1.7 meters tall, and weigh from 26-70 kilograms (males average 34-70 kgs, females average 26-50 kgs).  Skin color ranges generally from pale brown to black skinned, and their bodies are covered with black hair.  Juvenile chimps can usually be identified by paler faces and white tufts of hair on the rump, which they lose around the age of 5.  As they age, both sexes can develop graying and/or thinning hair.  The bonobo (Pan paniscus)[2], sometimes referred to as the pygmy chimpanzee, is now almost always recognized as its own species, though very closely related to the common chimpanzee.  Although it is smaller than the common chimp, the former name is thought to refer to the term used for Africans that share the bonobo range.

          Chimpanzees have arms that are about 1.5 times their height.  They are brachiators, meaning they are able to travel by swinging across tree limbs and branches.  Their long hands and fingers and short thumbs, able to curl around branches, are useful in this mode of travel.  Chimpanzees are also highly terrestrial. They are generally knuckle walkers, but individuals occasionally walk bipedally, often during displays of aggression or intimidation.

            Chimps live to 40-60 years of age.  During the rainy season, they may suffer from respiratory diseases, and at any time may be plagued by conditions also suffered by humans, such as gastrointestinal problems, skin ulcers, goiters, and osteoarthritis.  A polio epidemic spread to and decimated the chimp population at the Gombe Stream National Park in the 1970s.  These diseases, as well as infections resulting from injury, can be lethal.

            Chimpanzees are omnivorous, but eat mainly fruit and vegetation.  Researchers have found that chimpanzees prey upon other monkeys for meat; chimp males cooperatively hunt larger primates, such as colobus monkeys and baboons, as well as other mammals. 

            Chimpanzees have high cognitive development.  It is widely known that wild chimpanzees modify tools; they use them as fishing sticks, napkins, sponges and hammers.[3]  Wild chimpanzees are also believed to use plants as medicines.  People indigenous to the areas in which chimps are found use the same plants that chimps do to treat illnesses such as stomachache, headaches, and parasitic infections.  To further support this theory, chimps have been observed using particular plants at times when the medicinal value of the plants would be high, such as times of the year when parasitic infections are common, or after an individual chimp has appeared to be ill. 

            In the wild as well as in captivity, chimps are known to acquire information as a result of experience.  They problem solve, can abstract and generalize, can plan ahead and remember past events.

 

Habitat and Range

          There are three recognized subspecies of the common chimpanzee, all of which are native to the Africa.  Pan troglodytes verus is found from Gambia to the Niger River, Pan troglodytes troglodytes is found from the Niger River to the Congo River, and Pan troglodytes schweinfurthi is found from the Northwest corner of the Democratic Republic of the Congo into Western Uganda and Tanzania.  Throughout this article, all three subspecies are collectively referred to as chimpanzees.

          Chimpanzee habitat is mainly tropical rainforest, mixed forest-savanna, and mountain forest.  When at rest, during night or day, individuals almost always rest in the trees.  Chimps make beds or nests, always newly constructed.  They are made from leaves or other forest materials, and can include a covering on top and a platform on bottom.  Other than mothers and their offspring, individuals generally sleep alone.

            Chimpanzees travel in groups which occupy a general home range.  The group travels, rests, and eats together, for the most part, although individuals sometimes travel at the fringe of the group—this is explained in more detail below.  Males tend to travel farther and more widely than females, except during a female’s estrus period, when she may range as far as males.

 

Reproduction

            Chimpanzees are polygynandrous (a type of polygamy in which a female mates with several males, each of which also mates with several different females), and do not have a breeding season.  A female can come into estrus at any time of year.  Estrus generally lasts for 36 days, and during that period her genitals swell and change in shape and color—this swelling is referred to as “tumescence.” 

            Maximally tumescent females are the most frequent copulation partners within a group.  While females may mate with any number of males, sometimes a male may attempt to control sexual access to a tumescent female through aggression toward the female or interested males, with coercion, or consortships. 

            Aggression toward the female is more common than aggression toward an interested male for several reasons: the male attempting to control the mating of the female can avoid a fight with the interested male, the female is less likely to copulate with other males in order to avoid punishment, and controlling the female rather than the male prevents a third male from mating with the female while the first two fight.

            Females are willing participants in consortships, during which a male and female migrate to the fringe of the group for several days, over which they frequently mate.  Often the male partner in a consortship is a preferred partner of the female, but one who does not have enough status within the larger group to ward off other interested (and higher ranked) males. 

            Sometimes an estrus female is the center of a smaller group of several males, in which she mates with all of them.  In these small groups, the males are generally not aggressive towards one another.  Estrus females are very interesting to male chimps.  These females are often more successful than non-estrus females at getting food from males, accessing group feeding sites, and getting along with stranger males when groups meet.

            A female generally gives birth to one offspring, and sometimes two, following a 230-day gestation period.  In the wild, a female may first give birth at 15 years old.  In captivity females may give birth as early as 11 years old.  Once a female becomes pregnant, even if she does not ultimately deliver an infant, she may not cycle for another 2.5 - 5.5 years. 

            Chimps are independent at six years old, though ties between a mother and her offspring may last the entirety of their lives.  A juvenile depends on its mother for food and education.  Females provide most of the care, although siblings (both male and female) and aunts may “babysit.”  Even after offspring are weaned, they may travel with the mother until they reach puberty.  Females reach reproductive maturity at 10-13 years, and males at 12-15 years.

 

Social Structure and Behavior

            Chimpanzees are highly social animals, and can easily tell each other apart, even after not having seen an individual for years.  Some chimps who have learned American Sign Language have remembered an individual and the name sign for that individual after years had passed.

            In the wild, chimpanzees live in a “fission-fusion” society, in which individuals belong to a large community, but tend to travel, rest and eat with smaller groups.  The composition of these subgroups is fluid, and individuals may migrate in or out at any time.  Chimpanzee social groups have a clear, though changeable, social structure, in which there is one alpha male.  Male social status is often linked to his mother’s social status within a group.  Males may also change their social status over time by challenging higher ranking males.

            Coalitions between two or more individuals are freely formed within a group.  Often a coalition consists of two males, and usually is strongest when the purpose is to unseat a higher ranking male.  However, it is not uncommon for males to solicit high ranking females as coalition partners.  Coalitions change over time, as a result of death, migration, and changes in social status.

            In the wild, the strongest and most frequent bond is between adult males.  It has been observed that wild chimpanzee males are four times more likely to groom one another than females.  Brothers are almost always allies, but unrelated males often form bonds with each other.  Females most often travel alone, unless in estrus or with dependent offspring.  In captivity, however, female bonds are much more common.  Adult males are the crux of a social group.  Males patrol the borders of the group’s range, either alone or in groups.  Males sometimes care for orphaned siblings, even if the care-giving male is fully mature. 

            Grooming is a central part of relationship formation and maintenance.  Grooming rids individuals of pests and dirt, but also plays a crucial role in cementing bonds and diffusing tensions.  Researchers have referred to chimps who often groom with specific partners as “friends.”  Friends often groom when at rest, travel together, and support one another in aggressive encounters.  During rest times, and following periods of tension, other physical contact, such as hugging and embracing, is very common.

            Relationships between groups are marked by intense violence and aggression.  Chimpanzees are the only animals, other than human beings, that are known to aggressively stalk and infiltrate a neighboring stranger group for the purpose of wounding or killing the individuals they encounter.  Chimp groups have exterminated stranger groups in their entirety.

            Lack of stimulation is blamed for a wide array of abnormal and sometimes self-destructive behavior in captive chimpanzees. Some of the more commonly observed behaviors found in captive chimps that are not found in wild chimps are hair-pulling, rocking, feces eating and urine drinking.  Researchers note that the presence of abnormal behaviors can decrease in accordance with various factors, such as social housing, access to a playground, and the time an individual has spent with its mother.[4]

 

Conservation Status

            The World Conservation Union (IUCN) lists the common chimp as Vulnerable, and possibly Endangered, under Red List criteria as of 2000.[5]  Some subpopulations have become small and highly fragmented.  Chimpanzees are hunted in many areas, in some areas of their range more intensely so than others.  The IUCN lists habitat loss and degradation as a result of agriculture, extraction and human settlement as the main threats to chimp population.

            However, the most immediate threat to the chimpanzee species is the intense bushmeat trade, which so heavily affects chimp populations in Africa that reports indicate they will soon become extinct in the wild, outside of protected reserves.[6]

            The bushmeat trade also threatens humans.  It is now believed that the slaughter and consumption of infected chimpanzees led to the human contraction of HIV and the Ebola virus.[7]



[1] Shefferly, N. 2005. "Pan troglodytes" (On-line), Animal Diversity Web at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_troglodytes.html

[2] Williams, A. and P. Myers. 2004. "Pan paniscus" (On-line), Animal Diversity Web at http://animaldiversity.ummz.umich.edu/site/accounts/information/Pan_paniscus.html

[3] Goodall, Jane, 1986.  The Chimpanzees of Gombe: Patterns of Behavior Boston: Bellknap Press of the Harvard University Press.

[4] A. Warniment and L. Brent, Abnormal Behavior in a Captive Chimpanzee Colony, Southwest Foundation for Biomedical Research, on-line at http://www.animalwelfare.com/Lab_animals/biblio/jo-9.htm

[5] 2006 IUCN Red List of Threatened Species, IUCN 2006, on-line at www.iucnredlist.org

[6] See more on bushmeat section below.

[7] Zoo Stuttgart, 2000.  Gorillas in the Soup, on-line at http://www.bushmeat-campaign.net/engsite/pdf/disease.pdf

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